The only common polyatomic ion that is a cation is the ammonium ion (NH₄⁺).

Here's why:

* Cations are positively charged ions, formed by the loss of electrons.

* Polyatomic ions are groups of atoms covalently bonded together that carry a net positive or negative charge.

* Ammonium ion is formed when a nitrogen atom bonds to four hydrogen atoms and gains a positive charge.

Let me know if you want to know more about polyatomic ions!